Process for developing electrostatic



United States Patent PROCESS FOR DEVELOPING ELECTROSTATIC IMAGE WITH LIQUID DEVELOPER No Drawing. Application October 31, 1956 Serial No. 619,399

8 Claims. (Cl. 117-37) This invention relates to electrostatic printing. More particularly, it relates to an improvement in the development of permanent visible images in electrostatic printing processes.

The process of electrostatic photography is of fairly recent origin and is described in numerous patents issued to Chester F. Carlson, including United States Patents 2,297,691 and 2,357,809. As therein disclosed the process essentially comprises production of a latent electrostatic image and development of a visible image therefrom.

The latent electrostatic image may be produced by any of a number of procedures recognized in the art. For example, a sensitive element may be produced by coating a support with a suitable photoconductor, such as those described in the Carlson patents above noted, and imparting a uniform charge to said sensitive element by means described in Carlson, e.g. by a corona discharge, and finally exposing said charged sensitive element to a pattern of light and shadow illumination whereby the illuminated areas are discharged and a charge image remains which corresponds to the pattern presented to the sensitive element. Alternatively the charge may be rubbed on as described in the aforesaid Carlson patents. Still other methods of charging and of forming a latent electrostatic image have been described in copending applications, Serial Nos. 484,214 and 544,815, filed January 26, 1955, and November 3, 1955, respectively, by me as either the sole or joint inventor.

It has been suggested heretofore that instead of the methods for developing visible images by the use of a dry solid material, described in the aforesaid Carlson patents and generally known as either cascade development or powder cloud development, a method which did not involve the use of dry solid materials might present numerous advantages in the electrostatic photography process. One such method is disclosed in Carlson Patent 2,551,582. The sharpness of definition obtainable in the latter system is obviously limited. Still other previous proposals have involved the preparation of a suspension of pigment in a suitable liquid vehicle. The present invention is directed to an improvement in the developers of the latter type in which the charged solid particles are carried into contact with the electrostatic image in a liquid medium instead of in air or other gaseous medium as in the Carlson patents, in which there is described a process comprising the formation of a latent electrostatic image, the attraction thereto of finely divided electrically charged mist particles and transfer of the solvent to a paper sheet to form a solvent image which is rendered visible by contact with a roller carrying a dye, soluble in the solvent image material.

In electrostatic photography, the terms positive development and negative development have acquired a specialized meaning and it is in the following sense that these terms are hereinafter employed.

(1) Positive electrostatic development of a charge image is defined as the deposition of electrostatically attractable or opaque particles on the charged areas of a high resistance layer in an amount directly proportional to the charge retained.

(2) Negative electrostatic development on the other hand is defined as the deposition of electrostatically attractable opaque particles on a high resistance layer bearing a charge or latent image in an amount inversely proportional to the charge retained.

I have now discovered a particularly advantageous method for securing either positive or negative development of the latent electrostatic image produced in accordance with the prior art and the means for achieving this. In my method I have discovered a developer composition consisting essentially of three constituents as follows:

(1) A particulate material which serves as the pigment;

(2) A liquid vehicle in which the pigment is suspended; and

(3) A compatible organic material which enhances the operation of the suspension in the developing process.

A wide variety of suitable pigments are known in the art. For example, in Carlson Patent 2,297,691 there are described numerous organic and inorganic materials, including talcum powder, aluminum bronze, carbon dust, and other suitable materials. The principal requirement is that the particulate material be electrostatically attractable. Other properties which are determinative of its utility are the particle size, the sign and magnitude of the charge present on the particle and the behavior of the particles in an electric field. For increased definition, fine particles are preferred, as would be the case in the production of continuous-tone copy. For line copy, somewhat coarser particles may be used. Particle sizes from 40 microns down to about 1 micron have been found suitable in my process. An optimum particle size of about 10 microns represents a compromise between sharpness of definition, cost of comminution and ability to remain in suspension.

When finely divided pigments are suspended or dispersed in a suitable liquid vehicle, it has been observed that the solid particles acquire a charge. With some of the pigments investigated, a preponderance of the particles acquire a charge of one sign, while with others, the number of positively charged particles is about equal to the number of negatively charged particles. For efiicient operation, it is preferable that the pigment selected be one in which substantially all of the particles acquire a charge of one sign.

Of the pigments investigated, charcoal made from hardwood has proven to be the most suitable in a liquid 7 medium. Suitable charcoal is commercially available in sizes down to 40 microns. The particle size of this material can be reduced to about 10 microns by milling for about 7 hours, and for many purposes a particle size of 10 microns produces the desired degree of resolution. Further reduction to particle sizes of about 1 micron in a super ball mill is required for the development of continuous-tone copy, but it has been found that charcoal which is black at 10 micron particle size tends to appear brown when reduced to 1 micron. Other pigments may, of course, be used to produce visible images in any desired color, provided the electroscopic material is not chemically or physically affected adversely by the liquid vehicle of the developer, or by other materials with which it comes in contact under the conditions of operation.

The liquid vehicle must possess a high dielectric strength and a resistivity of at least about 10 ohm centimeters. This requirement automatically eliminates water and most alcohols, such as those found suitable in the process described in Carlson 2,551,582. In addition,

the liquid vehicle should, if possible, be one which does not have an appreciable toxicity and which is not too inflammable. In addition, it is desirable to use a liquid which is neither malodorous nor highly colored. Finally, it is essential that the liquid vehicle be inert to the material which bears the latent electrostatic image to be developed.

I have found that mixtures of hydrocarbons sold commercially under the name Solvesso by the Esso Standard Oil Company possess in large measure the desired qualities. Specifically, Solvesso 100 with a flash point of 100 F. and a boiling point of between 315 and 350 F. and Solvesso 150 with a flash point of 150 F. and a boiling range of 365 F. to 415 F. have been found to be admirably suited for incorporation in the developer composition as the suspending medium. The composition of these two proprietary mixtures, as reported by the manufacturer, was:

In addition, Solvesso 100 contains 4% of aliphatics for each 96% of the above mixture of aromatics and Solvesso 150 contains aliphatics for each 90% of the above mixture of aromatics.

I have found that compositions of pigment and vehicle as above described are materially improved by the addition of the third essential ingredient in my composition, an ingredient which, when present in the composition in suitable proportions, has been found to materially improve the initial production of the desired visible image and which also substantially improves the permanence of the visible image. I have discovered a group of materials possessing a peculiarly desirable combination of properties which contribute to these two improvements. The materials found by me to be suitable are a family of organic chemicals which are solid at room temperature and are only very slightly soluble at room temperature in the aromatic liquid vehicles above described, but which melt when heated moderately to produce a liquid which is miscible with or dissolves in the aforesaid aromatic vehicles. When present in solid form in the developer, the additive material migrates to the same electrode that the preferred charcoal pigment particles migrate to and hence reinforces the development process.

' The third essential constituent in my developer possesses one additional outstanding property in that it produces a permanently sealed image of the pigment particles, when the pigment particles are transferred to a permanent record medium, such as an absorbent web of paper. This permanent image is developed by evaporation of the solvent for extended intervals at ambient temperatures or for shorter times at elevated temperatures sufiicient to fuse the additive, but below the temperatures at which the record medium will char or burn.

I have found that for the production of dispersions of positively charged particles in a liquid developer composition polyamides are particularly desirable, while for the production of dispersions of negatively charged particles in a liquid developer composition, shellac is useful. I have used polyamides produced by the Chemical Division of General Mills in accordance with US Patent 2,378,413 marketed under the name Versamids.

Of a large number of materials studied as additives to improve the liquid developer system consisting of solid opaque electroscopic powder particles in a liquid having the necessary resistivity, only polyamides were found to behave in themselves or in combination with colored pigments almost entirely as positively charged particles. Thus liquid developer compositions containing Versamids are particularly suited to negative development of latent images under positive charging conditions. Versamids Nos. 900 and 930 have been found to be particularly useful. These are solid thermoplastic resins having melting points of about 185 C. and C. respectively. Orange flake grade shellac, in contrast, behaved largely as negatively charged particles and hence was peculiarly adapted to the positive development of latent images under positive charging conditions.

The several constituents may be brought together in the improved developer composition in any of a number of ways. One of the best procedures found for obtaining the developer was to melt the polyamide, either Versamids No. 930 or 900 or a mixture of 930 and 900, in a small quantity of hot liquid Solvesso hydrocarbon, in which the polyamide appeared to be soluble in all proportions. A sufiicient amount of charcoal powder was added to the hot mixture to obtain a stiff mass. Upon cooling, the mass was readily crumbled and when reduced to small pieces, the mixture was passed through a 20 mesh Tyler Standard sieve. The material was then milled for several hours, at the end of which time the solid particles were reduced in size to below 10 microns. Additional Solvesso was added to the concentrate to dilute the composition to the desired consistency.

The following examples will serve to further illustrate the practice of my invention.

Example I Forty grams of Versamids resin No. 930 were added to seventy-five cubic centimeters of Solvesso No. 100. The mixture was heated until it was completely melted and homogeneous. Sixty grams of charcoal powder particles 10 microns in size were then added to the hot solution and mixed thoroughly. The resulting solid mass was permitted to cool and when cooled was crushed in a mortar and sifted through a 20 mesh screen. The material passing through the screen was added to two hundred cubic centimeters of Solvesso No. 100 and ball milled for 1 /2 hours. The solids of the resultant product had a maximum particle size of about 7 microns, with most of the material between 5 and 7 microns. The ball mill and mill balls were rinsed with another one hundred cubic centimeters of Solvesso No. 100 and the crinsings combined with the concentrate. About two and one-half liters of Solvesso were added to the mixture to dilute it to a total volume of three liters. The resulting composition when flowed over a surface bearing a latent electrostatic charge quickly produced a clear visible image with good resolution.

Example 11 Forty grams of polyamide resin was placed in a beaker with 75 cc. of Solvesso and heated on a hot plate until polyamide melted. Sixty grams of charcoal were added to this hot polyamide solution and mixed thoroughly. When cooled, the solid mass was crushed in a mortar and sifted through a 20 mesh sieve. The mixture was placed in the attritor with 200 cc. of Solvesso 150 and milled until the particle size was approximately 7 about 1 /2 hours. 100cc. of Solvesso was used to rinse attritor and steel balls. The prepared developing solution was poured through a 250 mesh sieve. The prepared developer solution was diluted to approximately 8 liters by the addition of Solvesso 150. When brought into contact with a surface bearing an electrostatic charge image, the composition produced a clear visible image in a very few seconds. The visible pigment image was easily transferred to a web of newsprint paper which absorbed the vehicle rapidly, and was found to be sealed within the pores of the paper after evaporation of the liquid vehicle into the surrounding atmosphere. In other experiments the pigment image was fixed immediately after transfer by heating the permanent record medium to about 124 C. by means of heated platens or rolls.

The foregoing examples are merely exemplary of the manner in which my novel development composition may be prepared and used and are not to be taken as limitative.

The proportions of the several ingredients are susceptible of wide variation dependent on the electrical properties of the charge image, the contrast required, the background permissible, and whether the visible image is to be permanently recorded or merely viewed on a temporary basis.

While I do not Wish to be bound by any specific theory of operation, I believe that the polyamide or shellac and the pigment material cooperate to produce the ultimate image and that the additive may form a coating on the pigment particles during the preparation of the composition in any of the several ways above described. Since the additive, when present in the form of solid particles, is readily attracted to a charged electrode, it will be appreciated that such a coating phenomenon would not impair the desired development of a visible image.

I have found that almost equal proportions of pigment and additive produce satisfactory results and that relative amounts of between 9:1 and 1:9 of pigment to additive produce visible images with varying degrees of definition and contrast when brought into contact with surfaces bearing latent electrostatic charge images with up to 500 volts of potential. These pigment-additive mixtures have been dispersed in liquid aromatic hydrocarbon vehicles in concentrations of between 1% and 20% by weight of the entire composition. The amount of liquid vehicle may vary from between the minimal amount required to produce a viscous developer and the maximum amount, necessary to produce an extremely thin tinted dispersion producing faintly visible images.

I particularly prefer compositions in which the pigment is a hardwood charcoal having a particle size between about 1 micron and 40 microns; the vehicle is an aromatic hydrocarbon mixture of alkyl benzenes and the additive is a polyamide and in which the proportions by weight of pigmentzvehicleradditive are about 60:2700z40 with the additive being essentially all tied to the pigment under the conditions of use, rather than being dissolved in the vehicle in substantial amounts.

Having now described my invention in accordance with the patent statutes, I claim;

1. In a method of electrostatic printing which comprises: preparing a liquid developer composition consisting essentially of a finely divided electrostatically attractable solid opaque pigment material having a particle size not greater than 40 microns and which is dispersed in a high resistivity organic liquid having a specific resistivity of at least about 10 ohm centimeters, and bringing said liquid delevoper composition into contact with a member bearing an electrostatic charge image whereby said charge image is converted to a visible image by selective deposition thereon of the solid charged particles in said dispersion, the improvement which comprises modifying said liquid developer composition by including therein an organic material which is soluble in said liquid at elevated temperatures and which is relatively insoluble in said liquid at temperatures of about C. and which is electrostatically attractable when in solid form and which comprises a solid particulate material of the group of materials consisting of polyamides and shellac.

2. A method of developing a visible image from an electrostatic charge pattern which includes preparing a liquid developer composition by (1) dissolving a thermoplastic material of the group consisting of polyamides and shellac in an aromatic hydrocarbon liquid having a high resistivity, (2) adding finely divided solid pigment material to said solution in the relative proportions of between 1:9 and 9:1 pigment material to thermoplastic material, (3) comminuting the resulting solid mixture, and (4) dispersing the finely divided product in an aromatic hydrocarbon liquid having a high resistivity; and applying the resulting dispersion to both the charged and uncharged portions of the surface of a member bearing an electrostatic charge image, 'whereby said dispersed solid particles deposit selectively on the portions of the surface bearing a charge and thereby produce a visible image of said charge pattern.

3. The method of claim 2 in which the pigment material consists of hardwood charcoal particles finer than 40 microns.

4. A process for developing a visible image of a latent electrostatic charge image on the surface of a high resistance member which comprises: applying a liquid developer composition to the surface of the high resistance member bearing the latent electrostatic image for a time suflicient to convert the latent image to a visible image, the liquid developer consisting essentially of finely divided opaque solid electrostatically attractable pigment material of which the particle size is not greater than 40 microns dispersed in a liquid vehicle having a specific resistivity of at least about 10 ohm centimeters and containing solid particles of an electrostatically attractable thermoplastic additive selected from the group consisting of polyamides and shellac, the amount of pigment in the developer composition being sufficient to produce a visible image faithfully reproducing the charge pattern in the latent electrostatic image and the relative amounts of pigment and additive being between 9:1 and 1:9.

5. The method of claim 4 in which the additive is a polyamide.

6. The method of electrostatic printing which comprises forming a visible image as in claim 4 and transferring the visible image to a permanent record medium.

7. The method of claim 6 in which the visible image is fixed in said permanent record medium by evaporating the liquid associated with the pigment material defining the visible image.

8. The method of claim 6 in which the visible image is fixed in said permanent record medium by heating said image and said record material to a temperature suflicient to fuse said thermoplastic material.

References Cited in the file of this patent UNITED STATES PATENTS 115,915 Walsh et al June 13, 1871 1,806,965 Clewell May 26, 1931 2,297,691 Carlson Oct. 6, 1942 2,638,416 Walkup et al. May 12, 1953 2,649,382 Vesce Aug. 18, 1953 2,784,109 Walkup Mar. 5, 1957 OTHER REFERENCES Atkinson et al.: Ferrography, Journal of the Franklin Institute, vol. 252, No. 5, November 1951, pages 373 to 377.

Me tcalfe: Liquid Developer for Xerography, Journal of Scientific Instruments, vol. 32, February 1955, pages 74 and 75. 

1. IN A METHOD OF ELECTROSTATIC PRINTING WHICH COMPRISES: PREPARING A LIQUID DEVELOPER COMPOSITION CONSISTING ESSENTIALLY OF FINELY DIVIDED ELECTROSTATICALLY ATTRACTABLE SOLID OPAQUE PIGMENT MATERIAL HAVING A PARTICLE SIZE NOT GREATER THAN 40 MICRONS AND WHICH IS DISPERSED IN A HIGH RESISTIVITY ORGANIC LIQUID HAVING A SPECIFIC RESISTIVITY OF AT LEAST ABOUT 109 OHM CENTIMETERS, AND BRINGING SAID LIQUID DEVELOPER COMPOSITION INTO CONTACT WITH A MEMBER BEARING AN ELECTROSTATIC CHARGE IMAGE WHEREBY SAID CHARGE IMAGE IS CONVERTED TO A VISIBLE IMAGE BY SELECTIVE DEPOSITION THEREON OF THE SOLID CHARGED PARTICLES IN SAID DISPERSION, THE IMPROVEMENT WHICH COMPRISES MODIFYING SAID LIQUID DEVELOPER COMPOSITION BY INCLUDING THEREIN AN ORGANIC MATERIAL WHICH IS SOLUBLE IN SAID LIQUID AT ELEVATED TEMPERATURES AND WHICH IS RELATIVELY INSOLUBLE IN SAID LIQUID AT TEMPERATURES OF ABOUT 20*C, AND WHICH IS ELECTROSTATICALLY ATTRACTABLE WHEN IN SOLID FORM AND WHICH COMPRISES A SOLID PARTICULATE MATERIAL OF THE GROUP OF MATERIALS CONSISTING OF POLYAMIDES AND SHELLAC. 